Pulverised coal injection lances are generally used to inject pulverised coal as a substitute to coke into a blast furnace. The pulverised coal is conveyed pneumatically through the lance and fed into an oxidising atmosphere in a tuyere, through which hot blast air is blown into the furnace. In order to completely burn the pulverised coal, the combustion reaction should begin as close to the lance tip as possible. So-called Oxycoal lances consist of an inner pipe for conveying the pulverised coal and an outer pipe, concentric to the inner pipe, for conveying combustive gas, generally pure oxygen. The presence of pure oxygen at the lance tip improves the combustion conditions so that the combustion reaction starts at the lance tip.
It has been found, however, that the flame at the lance tip is not stable and does sporadically go out. In some cases, the flame can automatically reignite without intervention. This can however not be guaranteed. If the combustion of the pulverised coal does not take place at the lance tip because the flame has extinguished, the pulverised coal and the oxygen are fed into the blast furnace, and complete burning of the pulverised coal cannot be guaranteed.
A number of solutions have been proposed in order to improve the burning efficiency at the lance tip, generally by improving the mixing of the pulverised coal and oxygen. For example, EP 1 060 272 describes that the burning of the pulverised coal can be improved and the flame maintained by providing a flow swirler between the coaxial pipes so as to impart a swirling motion to the oxygen fed to the lance tip. The effect of the flow swirler however depends very much on the structure of the lance. If the spiral angle is too deep, the oxygen is directed away from the pulverised coal and the burning efficiency is decreased. If the spiral angle is too shallow, the improvement of the burning efficiency is negligible.
It has also been suggested in EP 1 060 272, to provide the outer surface wall of the inner pipe with a plurality of dimples near the lance tip for reducing fluid flow resistance and for improving the mixing of the pulverised coal with the oxygen at the lance tip.
Although the above systems may, in certain conditions, be suitable for improving the burning efficiency, this effect is not guaranteed and there remains a risk that the flame is not maintained.